A valve is provided in each of flow paths for supplying fuel gas or oxidizing gas to a fuel cell and for exhausting the fuel gas or oxidizing gas from the fuel cell. In order to detect failure of these valves to open, for example, Patent Document 1 discloses a device comprising a first opening/closing valve which is placed on a fuel gas supply pipe connected to the fuel cell, a second opening/closing valve which is placed downstream of the first opening/closing valve along a direction of flow of the fuel gas, a first pressure sensor which detects pressure of the fuel gas between the first opening/closing valve and the second opening/closing valve, a stop-time open/close valve operating unit which closes the first opening/closing valve and then the second opening/closing valve when the fuel cell is stopped, a stop-time pressure memory which stores at least an output of the first pressure sensor when the fuel cell is stopped, and a failure diagnosis unit which compares the output of the first pressure sensor when operation is re-started after the fuel cell is stopped and the output of the first pressure sensor when the fuel cell is stopped stored in the stop-time pressure memory, and diagnoses whether or not at least one of the first opening/closing valve and the second opening/closing valve has failed. In this reference, it is described that, when the pressure value in a volume section defined by the first opening/closing valve and the second opening/closing valve changes to a greater value, it is judged that the first opening/closing valve has failed, and, when the pressure value changes to a smaller value, it is judged that the second opening/closing valve has failed.    Patent Document 1: JP 2004-95425 A